1. Field of the Invention
The present invention relates to a sound reinforcement system, and more particularly to a sound reinforcement system which can be suitably applied to small-to-medium conference rooms.
2. Description of the Related Art
When a person who is speaking and the audience are in the same room above a certain size, and the audience cannot hear sound made by the person who is speaking well only by real voice, the sound needs to be reinforced and made audible throughout the room.
In general, in the case where sound is reinforced, a person who speaks has to speak in front of a fixed microphone, or a person who is speaking carries a microphone so that clear sound can be picked up. When speakers are changed during, for example, a question-and-answer session, a person who asks questions has to move to a fixed microphone, or a microphone has to be moved to him/her.
In many cases, speakers concentrated at one point or arranged at dispersed locations on a ceiling are used to reproduce picked-up sound. However, in the case where speakers are concentrated at one point, picked-up sound is excessively reinforced in the vicinity of the speakers, and also, in the case where speakers are arranged at dispersed locations, picked-up sound is excessively reinforced in the vicinity of a person who is speaking. Thus, sound cannot be uniformly reinforced throughout a room.
In Japanese Laid-Open Patent Publication (Kokai) No. H09-65470, an acoustic system for use in temples is disclosed which reinforces sound picked up by a fixed microphone using speakers arranged at dispersed locations on the ceiling of a room, and sets the volume of the speakers to get smaller as they become closer to the microphone so that the total volume of real voice and reinforced sound from the speakers can be uniform throughout the room.
Also, a speaker's face direction recognizing method and apparatus is disclosed in Japanese Laid-Open Patent Publication (Kokai) No. H10-243494.
Also, in Japanese Laid-Open Patent Publication (Kokai) No. H11-055784, an indoor sound reinforcement system is disclosed which picks up sound made by a person who is speaking using a microphone array. By the use of the microphone array, a handsfree sound reinforcement system can be realized.
As described above, in the conventional sound reinforcement system, a person who is speaking has to move to a fixed microphone, or a microphone has to be moved to a person who is speaking.
Also, there has been proposed a method in which the volume of reinforced sound from speakers arranged at dispersed locations is controlled so as to make uniform the total volume of real voice and reinforced sound, but delays in the propagation of acoustic signals have not been taken into account.
Also, it has been difficult to reinforce sound of a plurality of channels due to a risk of howling.
In a sound reinforcement system in which sound picked up by a microphone is reinforced and output from speakers arranged at dispersed locations on a ceiling or the like, there may be cases where reinforced sound from speakers behind a listener is louder than reinforced sound from speakers in front of the listener depending on the positional relationship between a person who is speaking and the listener. In this case, the listener may feel discomfort.
For example, if the output levels of reinforced sound from speakers arranged on a ceiling are set to get higher as they become away from a person who is speaking, the sound reinforcement level is high at a location which sound cannot directly reach, i.e., a location away from the person who is speaking, and hence reinforced sound from behind a given listener is louder than reinforced sound from the person who is speaking (ahead of the listener). This causes the listener to feel discomfort since the sense of sight and the sense of hearing are inconsistent with each other.
Also, in a sound reinforcement system in which an input signal from a microphone is amplified and reinforced from speakers arranged in the same space such a conference room or a hall, sound from the speakers may pass to the microphone to form a closed loop, which causes howling.
To prevent such howling, howling is detected and the gain of sound reinforcement is manually or automatically decreased, or a howling canceller that estimates the transfer function of the closed loop and performs signal processing is used.
Also, in the indoor sound reinforcement system disclosed in Japanese Laid-Open Patent Publication (Kokai) No. H11-055784, sound made by a person who is speaking is picked up using a microphone array, reinforced, and output from a plurality of speakers into a room, and which decreases the gains of speakers in the vicinity of the person who is speaking so as to prevent sound emitted from the speakers from being picked up by the microphone array to form the closed loop when the directivity of the microphone array is directed toward the person who is speaking in the vicinity of the speakers.
Regarding the sound reinforcement system for use in a conference room, hall, or the like, there may be cases where microphones of two or more channels are used at the same time and in the same room due to the presence of a person who speaks and persons who ask questions. In such a case, a plurality of acoustic paths exist, and hence howling is likely to occur.
Referring to FIG. 1, a description will now be given of an example in which sound inputs from a plurality of microphones are reinforced. In this example, it is assumed that a plurality of microphones and a plurality of speakers are arranged at dispersed locations on a ceiling.
In FIG. 1, when a person A is speaking, a microphone of one channel is used. Specifically, sound made by the person A is picked up by a microphone MICa located in the vicinity of the person A, amplified, and reproduced from a speaker SPb away from the person A. As a result, even a listener away from the person A can hear the sound made by the person A at a satisfactory volume level.
If a person B starts speaking while the person A is speaking, sound is reinforced using microphones of two channels. Specifically, sound made by the person B is picked up by a microphone MICb located in the vicinity of the person B as well as the above-mentioned microphone MICa that picks up sound made by the person A, amplified, and reproduced from e.g. a speaker SPa away from the person B.
On this occasion, a closed loop is formed as shown in FIG. 1 because sound made by the person A is picked up by the microphone MICa, amplified, and reinforced from the speaker SPb, and the resultant sound-reinforced signal passes to the microphone MICb that picks up sound made by the person B, is amplified, and is reinforced from the speaker SPa located in the vicinity of the person A, and the resultant sound-reinforced signal passes to the microphone MICa located in the vicinity of the person A. When the gain of this closed loop is greater than 1, howling occurs.
Conventionally, to prevent such howling, the gain of sound reinforcement is adjusted by a special operator. Also, when the gain of sound reinforcement is decreased for the purpose of preventing howling, sound cannot be reinforced at a satisfactory level.
Further, signal processing using a howling canceller as described above has also been known, but this is not effective since the transfer function cannot be estimated where microphones of a plurality of channels are used, although this is effective in the case where a microphone of only one channel is used. Also, to accommodate a plurality of channels, a complicated and expensive system is required.